You will notice when your machine is working too hard. When you get to know it a bit; sewing different things from shirt cotton, to flat felled seams on jeans, to a single layer of stiffer leather. A 15 will largely limit itself when it comes to neelde size, thread, and what you can fit under the presser foot. A few layers of soft leather is fine on domestic machines in general, 3 millimeters are probably fine. I haven't pushed my machines more than on occation. My advice would be to sign up on
The leather worker net, there's a few guys there who know and use these vintage machines. These models are likely to behave better on thick leather and take larger size needles (for thicker thread). I know some of them keep domestic straight stitchers like the 15.
I don't know about the potted version, but I have seen model 15 and model 66 used and abused for way to long and they have held up. I can't recommend over loading any machine, but they cleaned up well and just kept on running. These were belt driven machines and they are not meant to run for hours and hours with out a rest. I think someone adviced to use up a bobbin, then take a cup of tea before continue quilting. It's good for the sewer as well as the machine.
I have a 201 I fixed up and it has turned out to be my favorite, I'm treating it nicely
An old worn 15 or 66 can be found for next to nothing, they go on the heap or get split up for parts all the time unfortunately. I would rather experiment with one of those.